Pyrophoric nanoparticles can spontaneously ignite and burn when in contact with atmospheric oxygen. Such nanoparticles contain metals that can react with oxygen gas in the atmosphere to form metal oxides in a natural oxidation reaction. This natural oxidation reaction is a spontaneous exothermic process that usually proceeds slowly; however, the oxidation reaction can induce pyrophoricity and proceed much more quickly by increasing the exposed surface area of the metal. For example, by producing particles in a very fine powder form, the surface area available to react with atmospheric oxygen is greatly increased and can be high enough to cause the particles to spontaneously ignite.
The disclosed nanoparticles can be useful for various thermal applications where heat release is desirable, for instance in hand warmers. The disclosed nanoparticles can also be used where oxygen- or air-free environments are needed. For example, the disclosed pyrophoric nanoparticles can be used in a shlenk line or other vacuum gas manifolds, instead of vacuum and/or inert gas, since the pyrophoric nanoparticles can quickly scavenge small amounts of air or oxygen present in the reaction vessel to ensure the reaction vessel remains oxygen/air-free. Additionally, the disclosed pyrophoric nanoparticies can be used as an inexpensive alternative in chemical syntheses (e.g., of pthalocyanine at low temperatures) where activated elemental nickel and copper are typically used as catalysts.
Pyrophoric nanoparticles are often produced by including oxalate salts in a mixture and heating the mixture to produce metal-containing particles in situ. The resultant metal-containing nanoparticles can participate in a thermite (pyrophoric) reaction or serve as a catalyst. Typical oxalate salts include nickel- or copper-based type salts that can be very costly due to their high trading prices, for example. Current demands remain for both copper and nickel in other industrial uses so these metal prices will likely continue to escalate.
Yields for prior processes to achieve small particles, i.e., less than 50 nm, are typically less than 30%. Low yields, in combination with high material costs, produce expensive nanoparticles and in turn increase the price of end products.
Thus, there is a need to produce pyrophoric nanoparticles that are less expensive, and for methods that deliver higher throughputs and yields than current methods.